Method and apparatus for a dynamic gain equalizer for an erbium doped fiber amplifier

Abstract

This invention is a dynamic gain equalizer for fiber optic communication systems with a spectral output that can be changed dynamically as needed by adjusting one or more variable power lasers providing optical pumping. The proposed dynamic gain equalizer can adjust power inequalities in WDM signals resulting from the gain properties of EDFA's. The dynamic gain equalizer uses erbium ions in a system that has not been pumped by a laser, or pumped only to a small extent, with the result that passing radiation can be absorbed by the erbium ion. For a given erbium doped fiber, with a given fiber length, the spectral absorption of the fiber is dependent on the signal power entering the erbium doped fiber for each of the signal wavelengths. In order to vary the absorption spectrum of the filter, the erbium doped fiber is pumped by a laser at a low power, to levels where the excited state population inversion either has not occurred or is insufficient to achieve total gain. By choosing the adequate type of Erbium fiber and by altering the pump level of the dynamic filter it is possible to reach a very large dynamic range of gain for a given EDFA This filter can be used as a stand alone device or in conjunction with a dichroic filter, or any other filter, and can be an important contributor for dynamic gain equalization in erbium doped fiber amplifiers.

What is claimed is:  
     
         1 . A method of adjusting powers of signals that are carried substantially simultaneously on an optical fiber transmission line and that are amplified by a doped fiber amplifier that includes a first dopant at a certain concentration, comprising the steps of: 
 (a) optically coupling at least one attenuating fiber to the transmission line, so that the signals traverse said at least one attenuating fiber; and    (b) pumping said at least one attenuating fiber at a pump power insufficient to achieve total gain.    
     
     
         2 . The method of  claim 1 , wherein said pumping is effected at a pump power insufficient to achieve population inversion in said at least one attenuating fiber.  
     
     
         3 . The method of  claim 1 , wherein said pumping is effected at a pump power sufficient to alter an absorption spectrum of said at least one attenuating fiber.  
     
     
         4 . The method of  claim 1 , wherein said at least one attenuating fiber is coupled to the transmission line upstream from the doped fiber amplifier.  
     
     
         5 . The method of  claim 1 , wherein said at least one attenuating fiber is coupled to the transmission line downstream from the doped fiber amplifier.  
     
     
         6 . The method of  claim 1 , further comprising the step of: 
 (c) passively attenuating the signals.    
     
     
         7 . The method of  claim 1 , wherein said pumping is operative to substantially equalize the powers of the signals.  
     
     
         8 . The method of  claim 1 , further comprising the step of: 
 (c) doping each of said at least one attenuating fiber with a respective dopant.    
     
     
         9 . The method of  claim 8 , wherein, for at least one of said at least one attenuating fiber, said respective dopant is selected from the group consisting of erbium, aluminum, germanium, lanthanum, ytterbium and thulium.  
     
     
         10 . The method of  claim 8 , wherein, for one of said at least one attenuating fiber, said respective dopant is the first dopant.  
     
     
         11 . The method of  claim 10 , wherein said one of said at least one attenuating fiber is doped with the first dopant at a concentration different from the concentration of the first dopant in the doped fiber amplifier.  
     
     
         12 . The method of  claim 8 , wherein, for one of said at least one attenuating fiber, said respective dopant differs from the first dopant.  
     
     
         13 . The method of  claim 1 , wherein said pumping is effected using at least one laser.  
     
     
         14 . The method of  claim 13 , further comprising the step of: 
 (c) cooling at least one of said at least one laser.    
     
     
         15 . The method of  claim 13 , further comprising the step of: 
 (c) pumping the doped fiber amplifier, using one of said at least one laser.    
     
     
         16 . The method of  claim 1 , further comprising the step of: 
 (c) sampling the signals in the transmission line;    said pumping being effected in accordance with said sampling.    
     
     
         17 . An apparatus for adjusting powers of signals that are carried on an optical fiber transmission line, comprising: 
 (a) an attenuating fiber, optically coupled to the transmission line so that the signals traverse said attenuating fiber; and    (b) an attenuation control pump laser for pumping said attenuating fiber at a pump power insufficient to achieve total gain as the signals traverse said attenuating fiber.    
     
     
         18 . The apparatus of  claim 17 , wherein said attenuation control pump laser is operative to pump said attenuating fiber at a pump power insufficient to achieve population inversion.  
     
     
         19 . The apparatus of  claim 17 , wherein said attenuation control pump laser is operative to pump said attenuating fiber at a pump power sufficient to alter an absorption spectrum of said attenuating fiber.  
     
     
         20 . The apparatus of  claim 17 , comprising a plurality of said attenuating fibers.  
     
     
         21 . The apparatus of  claim 20 , comprising, for each said attenuating fiber, a respective said attenuation control pump laser.  
     
     
         22 . The apparatus of  claim 17 , further comprising: 
 (c) a first amplification fiber, optically coupled to the transmission line so that the signals traverse said first amplification fiber; and    (d) an amplification pump laser for pumping said first amplification fiber at a pump power sufficient to amplify the signals as the signals traverse said amplification fiber.    
     
     
         23 . The apparatus of  claim 22 , wherein said attenuating fiber is doped with a first dopant and wherein said first amplification fiber is doped with a second dopant.  
     
     
         24 . The apparatus of  claim 23 , wherein said first dopant and said second dopant are identical.  
     
     
         25 . The apparatus of  claim 24 , wherein said first dopant and said second dopant include erbium.  
     
     
         26 . The apparatus of  claim 24 , wherein said first dopant and said second dopant are present in their respective fibers at different concentrations.  
     
     
         27 . The apparatus of  claim 23 , wherein said first dopant and said second dopant are different.  
     
     
         28 . The apparatus of  claim 23 , wherein said first dopant and said second dopant are selected from the group consisting of erbium, aluminum, germanium, lanthanum, ytterbium and thulium.  
     
     
         29 . The apparatus of  claim 22 , wherein said first amplification fiber is optically coupled to the transmission line upstream from said attenuating fiber.  
     
     
         30 . The apparatus of  claim 22 , wherein said first amplification fiber is optically coupled to the transmission line downstream from said attenuating fiber.  
     
     
         31 . The apparatus of  claim 22 , further comprising: 
 (e) a second amplification fiber, optically coupled to the transmission line so that the signals traverse said second amplification fiber, said first amplification fiber being optically coupled to the transmission line upstream from said attenuating fiber, said second amplification fiber being optically coupled to the transmission line downstream from said attenuating fiber.    
     
     
         32 . The apparatus of  claim 17 , further comprising: 
 (c) a filter for passively filtering the signals.    
     
     
         33 . The apparatus of  claim 17 , further comprising: 
 (c) a tap for sampling the signals; and    (d) a processing unit for controlling said attenuation control pump laser in accordance with said sampled signals.    
     
     
         34 . The apparatus of  claim 33 , wherein said processor is operative to control said attenuation control pump laser to substantially equalize the powers of the signals.  
     
     
         35 . The apparatus of  claim 17 , wherein said attenuation control pump laser is downstream from said attenuating fiber.  
     
     
         36 . The apparatus of  claim 17 , wherein said attenuation control pump laser is upstream from said attenuating fiber.  
     
     
         37 . The apparatus of  claim 17 , further comprising: 
 (c) a amplification fiber, optically coupled to the transmission line so that the signals traverse said first amplification fiber, said pump laser being used both: 
 (i) to pump said attenuating fiber at said pump power insufficient to achieve total gain as the signals traverse said attenuating fiber, and  
 (ii) to pump said amplification fiber at a pump power sufficient to amplify the signals as the signals traverse said amplification fiber.  
   
     
     
         38 . The apparatus of  claim 17 , further comprising: 
 (c) a mechanism for cooling said attenuation control pump laser.